The present invention is directed to a printing unit with first and second screen-printing cylinders. Transfer cylinders cooperate with the screen-printing cylinders.
EP 07 23 864 B1 discloses a printing unit for a rotary printing press. This printing unit has a first screen-printing cylinder and a printing nip formed by two cylinders, in which a print stock is printed.
A screen-printing cylinder of this prior printing unit forms a printing nip in cooperation with a counter-pressure cylinder, which has circumferential sections, each with a reduced radius in which sheet grippers are disposed. On the interior of the screen-printing cylinder, a doctor blade device is provided, and is oriented toward the printing nip. This doctor blade device presses against the screen of the screen-printing cylinder and pushes ink through it. In order to prevent the screen from being deformed, when a circumference section with a reduced radius passes through the printing nip, a mechanism is provided which pulls the doctor blade device back from the screen at these times.
This prior art printing unit is only suitable for one-sided printing. The mechanism for retracting the doctor blade device is complex.
JP 11-129599 A discloses a screen-printing press with two screen-printing cylinders and a sheet-feeding transfer cylinder.
EP 0 949 069 A1 describes an offset printing press with cooperating transfer cylinders, which can be preceded by a screen-printing unit.
The object of the present invention is to provide a printing unit.
The object of the present invention is attained according to the invention by the provision of a printing unit with first and second screen-printing cylinders. Each screen-printing cylinder is associated with its own transfer cylinder. The two transfer cylinders cooperate to form a printing nip.
The advantages that can be achieved with the present invention are comprised particularly in the fact that it permits first forme printing and second forme printing, in the screen-printing process, with matching registers to be executed in a single printing procedure. The screen-printing cylinder is distinguished in particular by its uncomplicated construction.
In the printing unit in accordance with the present invention, at least one of the two screen-printing cylinders prints a first side of the print stock indirectly by use of a transfer cylinder. This transfer cylinder is the second of the two cylinders that form the printing nip and can therefore simultaneously serve as a counter-pressure cylinder for printing the second side of the print stock.
Preferably, the first of the two cylinders that form the printing nip is also a transfer cylinder. This results in an essentially symmetrical configuration of the printing unit of the present invention, with the same printing properties on both sides of the print stock.
In order to also permit an exact, register-matching printing, in relation to the edge of a sheet-like print stock, preferably at least one of the two cylinders that form the printing nip is equipped with a holding mechanism for the sheet-like print stock.
To be able to apply ink to the screen-printing cylinders, these screen-printing cylinders are each suitably provided with a doctor blade device on its interior, which doctor blade device is for use in pressing ink through a screen mounted on the screen cylinder. A device for preventing the doctor blade device from deforming the screen that it presses against is also advantageously provided in a specific circumferential section of the screen. This device prevents the screen from being excessively deformed when it is pressed against the holding mechanism or when it is pressed into a channel, which is complementary to the holding mechanism and which is disposed on the respective other cylinder forming the printing nip. This device thus prevents the screen from wearing too rapidly or from being damaged.
A device of this kind can be or can include an adjusting mechanism, which retracts the doctor blade device radially inward when the specific circumferential section of the screen-printing cylinder passes in front of the doctor blade device. In accordance with a second feature of the present invention, the screen deformation preventing device is a support element, which extends axially on the screen-printing cylinder in the vicinity of the specific circumferential section and serves to absorb a pressure exerted by the doctor blade in a radially outward direction on the specific circumferential section.
In order to prevent synchronization errors of the screen-printing cylinder, when the doctor blade device comes into contact with the support element and/or leaves contact with it, the support element is preferably provided with leading and trailing edges for the doctor blade device. These leading and trailing edges extend at an angle to a generatrix of the circumferential surface of the screen cylinder. Since the doctor blade device necessarily extends parallel to such a generatrix, the doctor blade device does not come into contact or leave contact with the support element over its entire span at one time, but only at certain points. In addition, braking forces acting on the screen-printing cylinder therefore remain low and are distributed over a finite section of the circumference of the screen-printing cylinder.